Project Euler

Problem 4: Largest Palindrome Product

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 x 99. Find the largest palindrome made from the product of two 3-digit numbers.

The Catch

How to check if a number is palindromic.

The Light

Parse a number to a character array. There are 2 possibilities:

The length of the array is even: from the 2 middle-indexed elements, check outward for equality.

The length of the array is odd: from the elements to the right and left of the middle-indexed element, check outward for equality.

The Code

public class Problem4
{
  public static void main(String[] args)
  {
    int max = 0;
    int value = 0;
     
    for(int i = 100; i < 1000; i++)
    {
      for(int j = 100; j < 1000; j++)
      {
        value = i * j;
        String s = Integer.toString(value);
         
        if(isPalin(s))
          if(value > max)
            max = value;
      }
    }
    System.out.println(max);
  }
   
  public static boolean isPalin(String s)
  {
    for(int i = 0; i < s.length()/2; i++)
    {
      if(s.charAt(i) != s.charAt(s.length() - 1 - i))
        return false;
    }
    return true;
  }
}